SWR is "Standing Wave Ratio" which is a measure of the antenna's match to the transmitter frequency. For an antenna to work well, it needs to be electrically the same length, or some major even fraction of the length, as the electromagnetic wave. With VHF centered around 157MHz, the wavelength is just about 1.9 meters, so an antenna needs to be 1.9m, .95m or .475m long for best performance. Shortening the antenna could easily result in a 75% power reduction. There are tricks to shorten the antenna physically while maintaining the proper electrical length with coils, but that's a manufacturing choice, not an adjustment that can be made.